Yeah, I use Google Wave. I honestly don't know what everyone else is talking about when they say they don't know what it's for.
Ian and I don't talk on the phone about Graphbug maybe as much as we should. There's a number of reasons for this, but as a result, we mostly communicate over Google Wave, which also serves as documentation for us.
It's better than email for communications that's consistently about something specific. We know what the other's doing code-wise, through pivotal and commit logs, and things outside of code, we update in a google wave. Or if we have a spec that we need to hammer out, it's better to do it through wave. It's more like a real-time wiki in this sense.
But we also use it to "Signing off" on informal agreements. We'll type up a set of bullet points and keep editing and arguing about it in the discussions below the main wave. The threaded nature of a wave keeps arguments and counter-arguments on target. And then when we're both in agreement, we sign the bottom of the top wave (with all the bullet points) and the date.
Because you can replay a wave, and that history doesn't change, we can be sure that neither one of us changed the bullet points after we've 'signed off' on the agreement.
Of course, there's nothing verifying the authenticity of the signature (meaning we can't verify who signed it), only that it's signed at the right time. There's probably a way, but for just us, just putting our names and date down is enough. Given that there's only two of us we know who signed what name.